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Full text of "Handbook Of Chemical Engineering - I"

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The High-temperature Scale.  The ideal temperature scale is known as the thermodynamic scale. Kelvin has denned this scale to be such that " the absolute values of two temperatures are to one another in the proportion of the heat taken in to the heat rejected in a reversible thermodynamic engine working with a source and refrigerator at the higher and lower of the temperatures respectively." The size of the unit degree is given by arbitrarily defining the temperature interval from the freezing point to the boiling point of pure water under standard conditions (760 mm. Hg. pressure) as equivalent to 100 Centigrade degrees.
No thermometer or pyrometer uses this principle of temperature measurement but scales defined by other means are supposedly in agreement with the thermodynamic scale or differ from this scale by small, known amounts.
Primarily the high temperature scale from 100C. to about 1,500C. has been established by the use of constant-volume nitrogen gas thermometer. If an "ideal" gas existed and were employed in the gas thermometer, the temperature scale would agree throughout this range with the thermodynamic scale. Such a gas does not exist, but the corrections which are necessary to apply to the nitrogen gas-thermometer scale to make this scale agree with that of an ideal gas are known. These corrections are small amounting to only 0.05 at 200C., 0.3 at 600C., and 1 at 1,200C.
The gas thermometer is difficult and inconvenient to operate and is not employed even for precision work except as a fundamental standard instrument. It would not be feasible to calibrate all other types of pyrometer directly by comparison with the gas thermometer. Accordingly a series of so-called fixed points have been adopted, denned by the melting or boiling points of several chemical elements and compounds. The temperatures at which these materials melt or boil have been determined up to 1,500C. by the gas thermometer. In these primary experiments a thermocouple or resistance thermometer served as a transfer instrument since it was not usually possible to place the bulb of the gas thermometer directly in the melting or boiling chemical element. Having obtained by means of the gas thermometer a number of fixed points which could be corrected to conform with the true thermodynamic temperature scale, it was found that the temperature scale defined by other forms of pyrometer agreed with the thermodynamic scale as closely as this scale could be determined by the gas thermometer, and that the precision possible in other pyrometers is far greater than that of which the gas thermometer is capable.
Thus, it has been found that the high temperature scale defined by the platinum resistance thermometer calibrated in terms of the melting point of ice, 0C., the boiling points of water, 100C., and of sulphur, 444.6C., at a pressure of 760 mm. Hg.,
1 U. S. Bureau of Standards.